Fragile magnetic ordering between robust 2D-ferrimagnets in the AFe3(SeO3)2F6 (A = K, Rb, Cs) series

Mixed anion compounds AFe(3)(SeO3)(2)F-6 (A = K, Rb) have been synthesized by hydrothermal reactions. Prior results obtained for the isostructural Cs analog have revealed original 2D-ferrimagnetic (FI) blocks, antiferromagnetically (AFM) ordered around similar to 120 K. Surprisingly, the whole series orders at a nearly constant T-N temperature with similar field-induced magnetization steps, despite significant changes of the interlayer thickness. The metamagnetic transition from AFM to FI on oriented single crystals and Cp measurements prove an in-plane easy-magnetic axis confirmed by neutron powder diffraction (NPD). The ferrimagnetic alignment [Fe(1)up arrow-Fe(2) down arrow -Fe(1)up arrow] in each 2D subunit is robust, but the AFM-ordering between them is broken under weak field leaving field-aligned 2D-macrospins, with weak magnetocrystalline anisotropy. DFT+U calculations validate the exchange ratio J(inter)/J(intra) of similar to 5.10(-3) for all compounds and the relatively high T-N value is well predicted by a modified random-phase approximation (RPA) like relation, established for Quasi-Low-Dimensional Heisenberg Antiferromagnets. However, our calculations of intralayer magnetic dipole-dipole (MDD) interactions indicate a non-neglectable values and suggest the latter to assist the in-plane spin orientation below T-N. At T-N, under field a 3D-Heisenberg model is proposed for the spin-flop AFM -> FI transition from the critical behavior using modified Arrott plots.

Automated summary

Mixed anion compounds AFe(3)(SeO3)(2)F-6 (A = K, Rb) with interesting magnetic properties have been synthesized and studied. Despite changes in interlayer thickness, the compounds exhibit consistent magnetic behavior at a nearly constant temperature. The magnetic properties were analyzed using calculations and experimental techniques, and a three-dimensional Heisenberg model was proposed to explain the transition from antiferromagnetic to ferromagnetic behavior.